1. FIELD OF THE INVENTION
The invention relates to a spectrophotometer, particularly double-beam or double-wavelength spectrophotometer.
2. DESCRIPTION OF THE PRIOR ART
A general double-beam spectrophotometer is constructed as follows. Rays of light emitted from a light source is introduced into a monochromator including dispersion elements. With rotation of the dispersion elements, the wave length of the light rays emanating from an exit slit successively changes. The monochromatic light from the monochromator is split into two beams by means of a rotating sector or a beam splitter. One of the beam split is called a reference beam while the other beam called a sample beam. A reference object is placed on the optical path of the reference beam and a sample, on the optical path of the sample beam. These light beams transmitted through the reference object and the sample are converted into corresponding electrical signals, by means of a photoelectric converting means. The electrical signal corresponding to the beam passing through the reference object or material is called a reference signal and the electrical signal corresponding to the beam passing through the sample is called a sample signal. In addition to those signals, a bias signal (usually called a zero signal) such as a signal caused by external light rays and a dark current is included in the output signal of the photoelectric converting means. In the case of the general double-wavelength spectrophotometer, rays of light emitted from a light source is introduced into two monochromators. The monochromatic lights from the monochromators are combined into a single light beam by a sector. The sample is placed on the light beam. The light beam transmitted through the material is converted into electrical signal. In carrying out the measurement, wavelengths of two monochromatic light rays may be fixed at different value, or one of the wavelengths may be fixed at a certain value while the other is scanned. The electric signal corresponding to one of the wavelengths is called a reference signal. The electric signal corresponding to the other is called a sample signal. In an ordinary spectrophotometer, the ratio of the reference signal R to the sample signal S, S/R, is calculated and the calculated one is displayed by a recorder. One of known methods for calculating the ratio feeds back a difference between a reference value and the reference signal R so as to render the reference signal constant. When an object to be under control of a feedback operation is the voltage to be applied to a photomultiplier tube, the method is called a dynode feedback method. When it is the slit width of a monochromator, the method is called a slit servo method.
When the sample signal S is larger than the reference signal R, the sample signal saturates the detector or an amplifier, frequently resulting in failure of the photometory. In this case, it is almost impossible to measure a negative absorbance in particular, and a wide range of photometering is impossible. By the narrow range of photometering, it is difficult to measure a change amount in the difference spectrum measurement. The saturation of the monochromator or the amplifier reduces the life time of the apparatus, or spectrophotometer. The zero signal is superposed on the reference signal R and the sample signal S and under this condition the feedback operation is performed so as to keep only the reference signal R constant. For this reason, with increase of the zero signal Z, the value of the reference signal R detected is smaller than that of the real value thereof. Therefore, the feedback operation is carried out with further increased voltage applied to the photomultiplier or further widened slit width, possibly resulting in uncontrollable running of the apparatus.